1. Field of the Invention
This invention relates to a stock or hollow metal ingot used in the production of cylindrical forged steel articles such as pressure vessel, oversized ring material and the like, and more particularly to a method and an apparatus for manufacturing a hollow steel ingot.
2. Related Art Statement
As a method for manufacturing a hollow steel ingot for use in the production of cylindrical forged steel articles and the like, there have been proposed a method in which a solid core made of metal or sand is coaxially set in a hollow cylindrical mold and molten steel is poured into an annular casting space between the mold and the core by top or bottom pouring process to cool and solidify it, a method of manufacturing a hollow steel ingot by centrifugal casting technique, which is entirely differend from the above, and so on. However, these methods have such problems that the arrangement of the core is complicated, the surface condition of the steel ingot is poor, molten steel at the side of the core is insufficiently cooled to cause a large segregation, and the like. As a result, hollow steel ingots to be sufficiently satisfied have not yet been obtained.
As a technique for solving the above problems, there has recently been proposed a method of manufacturing the hollow steel ingot wherein the ingot is constructed of a metallic cylinder used as an outer tube contacting with molten metal and a hollow or solid metal arranged inside the cylinder, and a cooling medium, such as air, water vapor or the like is flown therebetween (British Pat. No. 520598). Further, there has been proposed in Japanese Patent laid open No. 54-117,326 a method of manufacturing the hollow steel ingot, wherein a core constructed of a cylindrical steel tube and a cylindrical refractory member contacting with the inner wall of the steel tube is arranged in the center of the mold mounted on a stool, and molten metal is poured into the gap between the mold and the core.
These well-known methods make the arrangement of the core simple and improve the cooling of molten steel near the core, and consequently a lot of problems have been solved. However, for example, in the technique proposed in British Pat. No. 520598, there is a fear that the metallic outer tube contacting with molten steel is burned-out by molten steel flow in the pouring of molten steel. Once it is burned-out, molten steel is penetrated into the core, which makes the use of resulting hollow steel ingot impossible. On the other hand, if the thickness of the metallic outer tube is increased or the cooling is strengthened, cracks are produced over the inner surface of steel ingot due to the application of stress to the solidified shell in the solidification shrinkage of molten steel. The crack produced over the inner surface of the hollow steel ingot is unfovarable because it adversely affects products after the forging. Although it is certainly effective to use water, steam, liquid metal or the like in order to increase the cooling of the core, not only the equipment becomes complicated, but also the operation is very difficult. While, if a gas which is simply available is used as a cooling medium, the sufficient cooling is not still obtained in the well-known ordinary technique.
And also, the technique disclosed in Japanese Patent laid open No. 54-117,326 has such characteristics that cracks due to solidification shrinkage are not produced over the inner surface of the steel ingot, and even if the cylindrical steel tube is burned-out, the core has no problem in the structure and can simply be taken out after the solidification of molten steel, which solve many problems included in the conventional method of manufacturing hollow steel ingots. In this technique, however, the inverse V-shaped segregation produced in the steel ingot is not completely overcome, so that there may be still caused a problem that the inverse V-shaped segregation lines are produced on the inner surface of a product in the machining after the forging to spoil the quantity of the product.
In short, it is a main cause of these problems that the products produced from the hollow steel ingot recently become larger and the quality thereof is required to be higher. In practice, the problems included in the above prior art are fatal and it is practically difficult to manufacture hollow steel ingots of desired high quality and large size.